A winding, formed from a generic conductor, of an oil-filled transformer or similar electrical device having a relatively high power rating generally has cooling channels passing through it for the purpose of guiding a cooling liquid such as oil in order to dissipate heat which is incident owing to resistive losses. The heat produced in the conductor in this case needs to pass through an insulating sheathing, which is generally applied directly to the conductor and is typically produced from paper, and a boundary layer formed on a surface of this sheathing. In the process, a temperature gradient occurs both in the sheathing and in the boundary layer and there is thus a temperature difference between the conductor and the cooling liquid. At a given value for the heat to be dissipated, this temperature difference is a decisive variable for dimensions to be selected for the conductor and the corresponding winding.
In order to ensure sufficiently effective cooling, in particular larger dimensions therefore often need to be selected for windings having conductors according to the prior art than would be desirable. The documents EP 0746861 B1 and EP 1079500 A1 have disclosed conductors for transformer windings which comprise a plurality of conductor elements and have a perforated or net-like sheathing such that the corresponding cooling liquid can flow through the sheathing and, as a result, flow around the individual conductor elements. Although this achieves an improved cooling effect, this is at the expense of considerably impaired insulation of the respective conductor, in the case of which, instead of the conductor being insulated as a whole, only the individual conductor elements are insulated by a thin enamel coating. It is therefore only possible to use such conductors at relatively low voltages up to approximately 25 kV, since higher voltages require the conductor to be completely closed off by a sheathing.